The present invention provides a self-centering system for the structural frame of a building, in particular, the present invention provides a tension brace that provides elastic restoring forces to the frame of the building.
After an earthquake, permanent damage in even properly designed structures may allow for a building to come to rest in an out-of-plumb condition, which is often referred to as “residual (horizontal) displacement”. At best this can lead to loss of use (red tag) and at worst complete collapse in subsequent aftershocks. In recent years state-of-the-art earthquake resistant design research has focused on systems that are capable of ensuring that after an earthquake a building would have little, if any, residual displacement, thus they are “self-centering”. The proposed invention is such a system.
Various types of self-centering systems have been proposed. Inherent in the self-centering philosophy is the need for something in the structure to be pre-stressed and implemented into the building design in such a way that if a building is distorted during an earthquake, the pre-stressed element acts to pull the building back into alignment with its original position when the shaking stops. The pre-stressing allows the element to keep pulling with more than sufficient force all the way back to its original position, wherein the element is still pre-stressed at the conclusion of returning the structure back to its original form. For this to happen the pre-stressed element must remain elastic (unyielding) even as it is stretched under the combined demands of the pre-stressing and the supplemental strain induced by building deformations during an earthquake, and it is in meeting these combined strain demands by stretching without yielding that many of the difficulties of designing such systems are encountered. The present invention addresses these combined strain demands in a novel way, and at the same time allows a self-centering system to be designed that alleviates some of the additional challenges created in alternate earthquake-resistant building designs with self-centering systems.
Supplemental to the design of a self-centering system is the need to provide for dissipation of the seismic energy imparted into the building during an earthquake. The elastic, pre-stressed elements of the self-centering system, much like rubber bands, stretch and return to their original length without dissipating much energy. This is because the restoring force elements must be displaced only elastically so that they can recenter the building. Accordingly, some form of additional energy dissipation is usually (although not always) also included into proposed designs for a self-centering earthquake resistant structure, quite often in the form of a metallic yielding device of some kind. While there are many ways this could be accomplished (and with things other than metallic yielding devices, such as viscous dampers or fictional dampers), in the preferred embodiment of the proposed invention, the metallic yielding devices of U.S. Pat. Nos. 8,001,734 and 8,375,652 and US Patent Publication 2011/0308190 A1, would be incorporated into the system to fulfill the role of providing added energy dissipation. U.S. Pat. Nos. 8,001,734 and 8,375,652 are incorporated herein by reference. The disclosure of US Patent Publication 2011/0308190 A1, application Ser. No. 12/967,462 is also incorporated herein by reference.
Many types of earthquake resistant buildings with self-centering systems have been proposed and are being researched. The various types of geometry proposed for buildings with self-centering systems include: tall rocking braced frames that dissipate energy through the rocking movement of the frame and which are anchored at their tops by elongated steel tendons to provide self-centering; moment-resisting frames with gap opening behavior at the beam-column joints to dissipate energy and long tendons that connect multiple post and beam joints to self-center the building; and more standard braced-frame buildings that use diagonal braces made with superelastic alloys (shape memory alloys) to provide self-centering.
One of the challenges of self-centering systems is configuring the geometry of the building and the self-centering system in such a way that the strain induced in the elastic restoring force elements that provide the force necessary to pull the building back to its original position (this is called the “restoring force”) does not exceed the yield strain for the materials from which the elastic force restoring elements are made. As noted above, the elastic restoring force elements need to remain elastic throughout any design elongation or stretch so that they can return to their original geometry and bring the building back to its original position.
The amount of stretch that can be tolerated in typical, widely available materials, such as steel, before they begin to yield can be calculated from the equation
                    F        y            ·      L        E    ,where Fy is the material yield stress, L is the length of the material available to stretch, and E is the modulus of elasticity for the material. By way of example, a steel rod with a yield stress of 100,000 psi and a length of 10 feet can only stretch 0.4 inches before it begins to yield. That is to say, before it plastically deforms and then is unable to return to its original length or shape. If that rod is comprised of material with a yield stress of 200,000 psi, the rod will begin to yield after 0.8 inches of stretch.
As a result, designers who have wanted to use commercially available materials such as steel in their restoring force elements have focused on building designs that use long restoring force elements that need only elongate a small amount under the earthquake design load and thus do not elongate past their yield point. Such designs typically involve high aspect ratio geometries as with rocking frames, or with moment resisting frames designed with gap opening behavior at the beam-column joints with tendon rods spanning many beam and column joints to provide sufficiently long tendon rods. One researcher, Alan Jamal Stewart, has found that tendon yielding and loss of elasticity can still be a problem with moment-resisting frames designed with gap opening behavior at the beam-column joints.
Self-centering moment frame systems that rely on the gap-opening behavior of beam-to-column joints to dissipate energy also have a problem in their design that may interfere with their adoption. The structure is built with everything in its original condition and the as-designed fixed dimensions of the floor. Under lateral loading the gaps that form at the tops and bottoms of the beams at beam-column connections essentially pry the structure apart. The bare steel frames may not have a problem by themselves, but the building also contains a floor diaphragm system that is delivering the seismic inertial forces to the frames that are resisting lateral load, and this floor diaphragm does not want to be spread apart. While initially this was considered an academic problem in that it was mostly theoretical, the Feb. 22, 2011, Christchurch, New Zealand earthquake has now provided real-world examples of why this is a problem.
Self-centering systems that rely on high aspect ratio rocking frames also create a unique set of problems in addition to the possible yielding of the restoring force elements mentioned above with respect to moment resistant frames that rely on gap opening. High aspect ratio rocking frame systems work by allowing the (multi-story) frame to rock on the foundation under the effects of lateral loading. At each end of the rocking frame (or somewhere along its lateral width) elastic restoring force elements run from the foundation to the top of the frame to try to pull the side that has lifted back down into contact with the foundation. Aside from the detailing challenges of providing for this controlled vertical slip at the column bases, the actual shear forces carried by the frame must still be transferred to the foundation through a connection designed to do this while not compromising the rocking behavior of the frame. However, perhaps the biggest challenge with rocking self-centering frames is designing the interface with the surrounding structure. Somehow the building must be able to “push” on the frames while not interfering with the rocking mechanism of the frame. And gravity load must either be prevented from acting on the frame, or the connection of gravity load carrying members to the frame must be carefully considered in the overall design, taking care to not restrict the rocking mechanism of the frame either by placing too much gravity load on it or creating structural stiffness in the surrounding framing that prevents the desired rocking behavior.
The proposed invention aims to solve these problems by going back to a conventional bracing geometry. Conventional bracing geometry typically does not work because the axial strain demands (stretch) imposed on the braces would be too large to accommodate without yielding; however, the benefits to this approach is that the frame no longer has to rock, avoiding the associated problems of rocking frames, and the beam-column joints do not have to allow for gap opening as described above in the self-centering moment frame system. The proposed invention effectively increases the available stretch length of a tension brace in an otherwise fixed geometry.
FIG. 15 shows a schematic of a typical three-story braced frame structure where the self-centering elastic resisting force elements of the present invention could be used. The frame is configured such that the columns are 20 feet apart horizontally, and the beams are 14 feet apart vertically. FIG. 16 is a close up of any one of the brace-beam intersection points and shows, with solid lines, the undeformed shape, and in dashed lines the deformed brace geometry if three inches of interstory drift having been imposed on the story. Three inches interstory drift is a typical design parameter for braced frame structures. Measuring the change in length of the brace (being stretched in tension) we see that it must elongate 1.8″ to accommodate the interstory drift of three inches. The length of the brace (theoretically point to point for this discussion, but in reality a bit shorter due to the physical dimensions of the beam, column and connections) is 206 inches. In order for steel member that is 206 inches in length to stretch 1.8 inches without yielding, the yield stress of the steel would have to be at least 253,400 psi, which is not feasible. Additionally, as mentioned previously elastic restoring force elements (in this case the inclined braces), need to be prestressed in order for them to exert a centering restorative force, which would drive up the required yield stress by the selected amount of prestress.
The present invention provides an elastic restoring force element that has a primary length on the order of 206 inches, but effectively creates an elastic restoring force element that has a stretch length several times that.